Amplitude Shift Keying (ASK) is a type of amplitude modulation which represents binary data in the form of variations in the amplitude of a signal. The modulated signal has a high frequency carrier. A special case of ASK is known as On-OFF Keying (OOK) which uses only two amplitude levels. The binary signal when it is ASK modulated gives a zero value for a Low input while it gives the carrier output for High input. The ASK modulated signal upon receipt is demodulated by an ASK demodulator (or receiver). ASK can be generalized depending on how many possible levels of amplitude the signal can have to represent the data. Another known pulse amplitude modulation is 4-PAM which has 4 amplitude levels to represent the data.
There are two types of pulse amplitude demodulation techniques, being Asynchronous Demodulation/detection and Synchronous Demodulation/detection. When the clock frequency at the transmitter becomes substantially close to the clock frequency of the receiver, it is known as a synchronous detection method, as the frequency gets synchronized. Otherwise, the demodulation is known as asynchronous detection.